Correct Mesh Deformations?
Polygons with more than 4 sides (ngons) do not subdivide well.

Instead of one big ngon on top like this:



try to fill your mesh with quads:



First delete the faces that have more than 4 vertices:



Add a few loop cuts (CtrlR) that match the vertices on the joints of your mesh.



Then fill the gaps making quads.



Your mesh should be nice and smooth now. Even with smooth shading or a Subdivision surface modifier.